1. Technical Field
This invention relates to the control of toner concentration used in electrophotographic marking engines and more particularly, in using the signal strength of the toner monitor to accurately locate each developer station with respect to a stationary photo-conductor.
2. Background Art
With the development of electrostatographic marking engines using more than one color, the need arises to monitor and control the toner concentration in more than one development mixture. In an effort to minimize manufacturing costs, considerable engineering effort continues on developing cost effective solutions at uncompromised performance. To this end, it has been proposed that cost-effective control of toner concentrations in more than one development mixture can be accomplished by using only one toner monitor. See, for example, commonly assigned U.S. patent application Ser. No. 07/632,677, now U.S. Pat. No. 5,192,972, filed in the names of A. S. Kroll and W. Chang on Dec. 24, 1990.
U.S. Pat. No. 4,928,146, issued to Yamada on May 22, 1990 is illustrative of a number of references which show the development of a series of electrostatic images carried on a photoconductive drum with different colored toners at a single development position. See also U.S. Pat. No. 3,797,930, Tanaka et al, issued Mar. 17, 1974; U.S. Pat. No. 4,275,134, Knechtel, issued Jun. 23, 1981; Japanese Koki 1-244477 (1989); U.S. Pat. No. 4,728,983, Zwaldo, issued Mar. 1, 1988. A series of four development units are moved one after another to the development position. Each unit develops an image and is replaced by another unit as the series of units is indexed to apply a different color toner to the next image. The series of units are arranged side-by-side and moved linearly through a position in which the unit to be used is aligned with the development zone. When aligned or slightly before, a cam is rotated to push the entire unit toward the development zone, generally moving transverse to the motion of the series of units.
Other references such as U.S. Pat. No. 4,615,612 discloses color image-forming apparatus wherein a plurality of image developing units are supported on a turret which rotatably indexes the units to move a desired unit to a position for developing an image on a photoconductor. The position of a developing unit is detected and compared with a predetermined target position. A controller controls the speed and position of the turret for accurately positioning a desired developing unit. Data tables are used to stored target positions corresponding to constant time increments.
This general approach has the advantage of utilizing only a single development position for applying four different color toners to electrostatic images. This permits the use of development units whose size and number would prohibit them being spaced around the periphery of a relatively small photoconductive drum. It thus also permits the use of a small photoconductive drum. The use of a small drum has many advantages such as reduced expense and reduced size of the apparatus.
In most of these apparatus, a drum photoconductor is permanently fixed in the apparatus as the supporting structure for each development unit. With such structure, critical positioning of each development unit with respect to the photoconductive drum can be managed by precise manufacturing and assembly of those parts and their supporting structure. It would be desirable to remove the need for such precision.
U.S. Pat. Nos. 4,922,302, issued to Hill et al on May 1, 1990; 4,884,109, issued to Hill et al on Nov. 28, 1989; and 4,797,704, issued to Hill et al on Jan. 10, 1989, show a development station having an applicator with a rotating magnetic core and a stationary nonmagnetic sleeve around which a developer mixture is moved by rotation of the core to pass the developer through a development position. The applicator is fed by a rotating paddle positioned below the applicator which both mixes the developer and supplies it to the applicator. This particular structure requires that the applicator not be in contact with the image member carrying an electrostatic image to be developed, but that it be precisely spaced from it.
U.S. Pat. No. 4,801,966 is typical of a large number of references showing toning stations that are movable in an out of their own unique developer position to apply the correct color toner to the image being toned. Here, a developer applicator which is spaced from a photoconductive drum by a pair of rollers which engage the drum. This approach will provide accurate spacing only if other aspects of the relative position of the applicator and drum are precisely controlled.
Environmental parameters greatly impact the toner monitor readings, which is addressed in U.S. patent application Ser. No. 07/770.266, filed Oct. 3, 1991 in the name of Miller et al now U.S. Pat. No. 5,164,775. A reference block, which is made of a stable material used to compensate for the environmental parameters, requires accurate positioning. Each of the developer stations containing the development mixture require consistent and accurate positioning of the toner monitor, when a single toner monitor is used for multiple developer stations. A voltage signal received from the monitor varies as a result changes in toner concentration as well as to the location of the signal being measured. Variations in locating the toner monitor with respect to the developer stations and the reference block may reduce the validity of the measurements being received from the toner monitor. Statistically consistent measurements are necessary to obtain a functional system for measuring the toner concentration when implemented in a manufacturable product.